Plant & Animal Genomes XIV Conference

January 14-18, 2006
Town & Country Convention Center
San Diego, CA

The Genomic Organization Of Chromosome 6 In Medicago truncatula

Majesta Siegfried , Fares Najar , Bruce A Roe

  Department of Chemistry and Biochemistry Stephenson Research and Technology Center The University of Oklahoma 101 David L. Boren Blvd. Norman, Oklahoma 73019

Our laboratory is a member of the international consortium responsible for sequencing Medicago truncatula, a model legume, focusing on chromosomes 1, 4, 6, and 8. With over half of the euchromatic region of chromosome 6 sequence available, many interesting aspects to this chromosome are beginning to emerge. For example, chromosome 6 is rich in heterochromatin. Unlike the other seven Medicago chromosomes that have large blocks of euchromatin on each arm and heterochromatin near the regions at the ends and surrounding the centromere, chromosome 6 seems to resemble maize and the human Y chromosome, with the euchromatic regions interspersed with blocks of heterochromatin. In addition to these larger findings, we also are investigating the genomic organization of NBS-LRR genes, viral insertions, and repetitive sequences on chromosome 6 at the genic level. Thus far we have observed that chromosome 6 contains more NBS-LRR resistance genes than any of the other chromosomes, with 69 putative R genes predicted by similarity searches with an e value of at least e-10, with the majority of these genes occurring in clusters. Based on ClustalW alignments and phylogenetic comparisons, these clusters appear to have arisen by tandem duplications of an ancestral gene that then diverged over time. This divergence seems to be under selective pressure that maintains their function, but has allowed these resistance genes to acquired new substrate specificities.